[LazyLibrary] report full cycle when one is detected

masque/library.py

@@ -1267,12 +1267,12 @@ class LazyLibrary(ILibrary):
     """
     mapping: dict[str, Callable[[], 'Pattern']]
     cache: dict[str, 'Pattern']
-    _lookups_in_progress: set[str]
+    _lookups_in_progress: list[str]
 
     def __init__(self) -> None:
         self.mapping = {}
         self.cache = {}
-        self._lookups_in_progress = set()
+        self._lookups_in_progress = []
 
     def __setitem__(
             self,
@@ -1303,16 +1303,20 @@ class LazyLibrary(ILibrary):
             return self.cache[key]
 
         if key in self._lookups_in_progress:
+            chain = ' -> '.join(self._lookups_in_progress + [key])
             raise LibraryError(
-                f'Detected multiple simultaneous lookups of "{key}".\n'
+                f'Detected circular reference or recursive lookup of "{key}".\n'
+                f'Lookup chain: {chain}\n'
                 'This may be caused by an invalid (cyclical) reference, or buggy code.\n'
-                'If you are lazy-loading a file, try a non-lazy load and check for reference cycles.'        # TODO give advice on finding cycles
+                'If you are lazy-loading a file, try a non-lazy load and check for reference cycles.'
                 )
 
-        self._lookups_in_progress.add(key)
+        self._lookups_in_progress.append(key)
+        try:
             func = self.mapping[key]
             pat = func()
-        self._lookups_in_progress.remove(key)
+        finally:
+            self._lookups_in_progress.pop()
         self.cache[key] = pat
         return pat